This invention relates to apparatus for measuring fluid flows, of the kind in which a vortex-shedding bar, preferably symmetrical in cross-section with regard to the direction of fluid flow, is positioned to extend transversely in the flow stream, and the number and frequency of the Karman vortices induced by the bar and shed alternately from opposite sides of the bar is measured. From this measurement it is possible to calculate the main fluid stream velocity, and suitable means for doing this automatically is usually provided.
In the study of fluid flows, it is well known that the ideal flow of a non-viscous fluid past a body of circular cross-section is a laminar flow which is parted symmetrically as it meets the circular-section obstruction, passes around it, and then closes in again behind the obstruction as a symmetrical laminar flow. In practice no real fluid is without viscosity, and in consequence the local velocity of the flow in the immediate vicinity of the surface of the obstruction is lowered by the drag, as compared with the main stream. Thus at low Reynold's numbers, i.e. at low velocities for a given fluid and a certain size of obstruction, the flow pattern is modified to one of symmetrical cross-section but with two zones of elongated reverse rotation occurring in a "stagnation zone" immediately downstream of the obstructing body. As the flow velocity increases these elongated zones become increasingly symmetrical, until instability occurs and separate vortices form alternately on opposite sides of the body. These vortices, known as Karman vortices, stream rearwardly in a stable series alternately from opposite sides of the obstructing body.
If the fluid flow as a whole has a velocity V.sub.o, it will be found that the Karman vortices formed move at a velocity V.sub.v which is always smaller than V.sub.o. It can be shown (omitting the intermediate mathematics) that the time T between the successive shedding of vortices, the main fluid flow velocity V.sub.o, and a linear transverse dimension L of the obstructing body is related to the Reynold's number Re as follows: EQU T.multidot.V.sub.o /L=.phi. (Re)
For a cylinder of diameter D it is found that T.multidot.V.sub.o /D is approximately 5 for values of Re between 2.times.10.sup.2 and 2.times.10.sup.5.
It is usually more convenient to measure the frequency f of vortex shedding, when f=1/T, so that EQU f.multidot.D/V.sub.o .apprxeq.0.2 for a cylinder.
From this expression an empirical constant can be derived for a likely range of flow conditions and a fixed cylinder diameter. If this constant is K then EQU V.sub.o =f/K
Thus by measuring the frequency f of the Karman vortex shedding, it is possible to determine the main fluid stream velocity.
A similar state of affairs exists for bodies of other than cylindrical shape.
Various industrial flow-velocity measuring devices have been developed, based on measurement of the frequency of Karman vortex shedding. For example, the passage of the vortices downstream of the vortex-shedding bar may be detected, either by an ultrasonic sensing beam directed across the width of the flow at a crystal transducer, or by means of fine wires stretched locally across the flow stream, usually known as a hot-wire anemometer, and conducting a constant amount of electrical power, with means for sensing the changes in resistance of the wires with temperature; or in some other way. These means recognise the changes in local velocity in the flow as the rotating vortices pass by, and give an output signal whose shape tends towards the sinusoidal, the peaks (positive and negative) representing the passage of the vortices. As the signal voltages are small, they are amplified and the peaks are chopped, producing a train of square waves which is passed to a digital counter or frequency meter. As indicated above, the overall flow velocity V.sub.o of the flow being investigated, at the vortex-shedding bar, is directly proportional to the frequency f of vortex shedding from the bar, so that the measurement of f enables the flow velocity V.sub.o to be determined.